AVR-Chat

On 25-Jun-12 16:36, bayramdavies wrote:
> enkitec@... wrote:
>
>> I want to build a FM radio squelch using an ATMEGA8.
>> The ADC will be sampling the FM radio audio output
>> ... What is the best way of doing this in software?
> Mark, you don't need to do this with analog hardware unless the microcontroller has other time-critical stuff to do.  Sample the audio with the A-to-D as you already said, pass the samples through filters to separate high and low bands, compute the filter output magnitudes, smooth them and watch their levels over time.
>
> Others have advised you on the filters.  Personally, I would put the corner frequencies at about 250 Hz and 1000 Hz to start with and just see what I get.  A big advantage of using digital filters is that they are so easy to change!  I would use second-order IIR filters.  FIR filters take more computation for any given cut-off rate and you don't need their linear phase response.  There are plenty of filter design tools on the Web.
>
> Does this answer your question?
>
> Graham.
>


     I have used the low-pass filter described below, but I'm not sure 
on how to make it a high-pass:

----------------------------

A single pole (first order) high pass is pretty easy.
The basic building block for a lot of simple filters is a single pole 
low pass:

FILT <--- FILT + FF(NEW - FILT)

This algorithm is performed once for each new input sample.
NEW is the new input value for that iteration and FILT is the filtered 
output value.
FF is something I call the filter fraction. It adjusts how "heavy" the 
filter is.
FF = 0 is infinitely heavy since the filter output never changes.
FF = 1 just passes the input directly to the output without any 
filtering applied.
Obviously useful values are in between.
On some hardware it can be useful to make FF = 2**-N so that the 
multiply by FF becomes just a right shift by N bits.

You can make a high pass filter by subtracting the low pass filtered 
signal from the original.

----------------------------


     Does it mean that (NEW - FILT) is the high-pass filter output?

     Thanks.
     Mark Jordan